1. Field of Invention
The current invention relates to emergency medical equipment and treatment for cardiac arrest. In particular, the invention relates to cardiopulmonary resuscitation and cardiopulmonary circulatory assist devices that cyclically apply compressive pressure to a patient's thorax to increase intrathoracic pressure to force blood flow through the heart and other body organs.
2. Background of the Invention
More than one in four Americans have cardiovascular disease, which is the leading cause of sudden cardiac arrest (SCA) and a leading cause of death in the United States. It is estimated that each year in the United States, approximately 1.5 million people suffer a heart attack, resulting in more than 400,000 people experiencing an SCA episode, of whom at least 85% will die as a result. Approximately every minute there is a sudden cardiac arrest in the United States.
SCA is generally due to ventricular fibrillation, a life-threatening condition, in which the heart's normal electrical signals become chaotic, causing the cessation of effective pumping of blood by the heart. Blood carries oxygen to the body. If the blood flow stops, the body stops receiving oxygen. Irreversible organ damage will occur if blood flow is not restored promptly. The body, especially the brain and heart muscle, cannot survive beyond a few minutes without oxygenated blood flow.
The optimal first line of treatment for ventricular fibrillation, the most common cause of cardiac arrest, is defibrillation (the delivery of a high-energy electrical shock to the chest). Successful defibrillation stops the chaotic electrical activity and allows regular electrical activity to again produce normal heart rhythm. If the heart muscle has been deprived of oxygen by a lack of blood flow for more than a few minutes, defibrillation attempts are usually unsuccessful at restoring the heart to a normal rhythm. Similarly, the lack of oxygenated blood flow through the body will rapidly cause irreversible damage to vital organs including the brain. Medical defibrillation equipment is expensive, and dangerous in the hands of unskilled persons. Such equipment is usually only found in hospitals, doctors offices, and well-equipped mobile emergency units. Accordingly, medical defibrillation equipment is not often nearby when a patient suffers cardiac arrest.
Manual cardiopulmonary resuscitation (CPR) is most often used on cardiac arrest patients in the first critical minutes of cardiac arrest. Since 1960, manual CPR has been promoted as the standard means for providing oxygenated blood to the heart and brain until appropriate definitive medical treatment can restore normal heart and ventilator action. A primary objective of CPR is to generate blood flow to restore the heart to a condition that will allow successful defibrillation thereby causing the heart to resume normal beating. Another key objective of CPR is to provide blood flow to the brain and other organs to prevent irreversible organ damage while attempts at defibrillation are made.
Despite the widespread application of standard manual CPR, the average long-term survival rate from SCA is only about 15%, and may be lower if the SCA event occurs out of a hospital. Approximately 90% of the 350,000-400,000 persons who experience out-of-hospital sudden cardiac arrest in this country per year are served by local emergency medical teams whose directive is to initiate and maintain a resuscitative effort. The emergency teams transport patients to the hospital, but only 20% of individuals struck with cardiac arrest and who are treated by an emergency medical team survive to be discharged from the hospital. The poor survival rate for SCA is due, in part, to the inability of manual CPR to generate substantial blood flow, as well as the variability in the skill, strength, experience, fatigue or emotional state of the rescuer.
Most SCA episodes occur outside the hospital. The first medical professionals to reach a patient suffering from cardiac arrest are emergency paramedics. Upon arrival of an emergency medical vehicle, the emergency personnel applies a series of defibrillation attempts, which, if they fail to restart the heart, are followed by a rigorous procedure combining CPR with ventilation and additional defibrillation applications. If the heart muscle has been deprived of oxygen by a lack of blood flow for more than a few minutes, defibrillation attempts before administering CPR are usually unsuccessful at restarting the heart. Given that emergency medical response times to out-of-hospital SCA episodes are generally six to ten minutes, initial defibrillation attempts generally fail, thus necessitating that the combined routine of CPR and defibrillation be utilized. If the emergency medical vehicle is not equipped with a defibrillator, then the emergency response personnel generally relies on manual CPR to restart the victim's heart.
In manual CPR, a patient is placed on his back and the hands of the rescuer applying CPR are rhythmically pressed firmly against the center of the sternum on the patient's chest, i.e., thorax. By pressing on the sternum, the rescuer compresses the patient's thorax (chest cavity) to increase the pressure within the thorax and around the heart and intrathoracic vascular system. A primary aim of manual CPR is to increase intrathoracic pressure due to a decrease in thoracic volume produced by the displacement of the sternum. The rhythmic press and release by CPR of pressure around the heart forces blood to flow through the heart and the rest of the body.
A principal problem with conventional closed chest CPR is the inability to adequately produce sufficient blood flow to the brain and heart needed for survival. Animal studies have documented coronary and cerebral blood flows during CPR to be less than 5% and 10% of the pre-arrest values, respectively. Animal and human studies have determined that coronary perfusion pressure (CPP) is the best predictor of the success of myocardial recovery. During arrest, the coronary vasculature is believed to be fully dilated due to global ischemia (lack of blood flow and tissue perfusion). Accordingly, coronary blood flow should be directly related to the amount of CPP that can be generated with CPR. Studies indicate that a CPP of at least 15 mm Hg is required for successful myocardial resuscitation. If CPP is maintained at a level approaching 25 mm Hg, then many patients in cardiac arrest should be resuscitated. Restoration of coronary and cerebral perfusion flow are major determinants of the outcome of CPR. The duration of time during which the patient has no flow (from cardiac arrest to initiation of CPR) and the duration of CPR to return of spontaneous circulation (ROSC) are both crucial to the survival of the patient.
Chest compression occurs when the anterior and posterior thorax surfaces are moved toward one another, "flattening" the chest (anterior refers to the front of the thorax, posterior to the back). Chest compression is accompanied by an increase in the pressure within the thoracic cavity, and a decrease in the volume of the lungs. The decrease of the volume of the lungs is minimized by quickly trapping air in the lungs when starting compression of the chest. The increase in intrathoracic pressure forces blood through the heart and out toward the brain and extremities.
The amount of pressure needed to be applied to the chest for effective CPR is relatively great. Manual CPR often fails because inadequate pressure is applied to the chest by the hands of the person applying CPR. Moreover, the amount of force needed to achieve effective CPR is slightly below the force level which will traumatize the patient. Manual CPR often results in trauma to the patient's thorax because the person applying CPR applies excessive force to a small area on the chest in an effort to compress the heart. The most common injuries from manual CPR include injuries to the skin, bony thorax and upper airway. The reported incidence of injuries from CPR ranges from 21% to more than 65%. Accordingly, even properly executed manual CPR can lead to injury.
Applicants designed a vest-CPR system to increase intrathoracic pressure and intravascular pressure to produce blood flow using a continuous blower to directly pressurize the vest. The maximum output pressure of the blower corresponds to the desired peak vest pressure. The blower is a self-regulating source of vest pressure that does not require the complex and expensive regulators used in prior vest systems.
A CPR-vest is a belt that fits snugly around the thorax of a patient in cardiac arrest or requiring a cardiac assist. The vest includes a bladder underneath the belt and covering at least the front of the patient's thorax and preferably covers at least three fourths of the circumference of the chest. The bladder is connected by a pneumatic hose to an air supply and controller that rhythmically pressurizes and depressurizes the bladder. When pressurized, the bladder presses against the entire front of the thorax, from the armpits to the bottom of the rib cage to increase intrathoracic pressure.
By applying circumferential compression to reduce chest volume, vest-CPR increases intrathoracic pressure to increase the vascular pressure and force blood flow through the heart, lungs and other body organs. An initial rapid inflation of the vest bladder and corresponding increase in intrathoracic pressure traps air in the lungs to prevent excessive deflation of the lungs. By trapping air in the lungs, the continued inflation of the bladder results in fast increases of intrathoracic pressure with minimal inflation of the bladder because the trapped air in the lungs assists in increasing intrathoracic pressure. In addition, defibrillation electrodes may be positioned underneath the CPR vest to apply an electric shock while the CPR vest is operating.
Prior vest-CPR systems, such as shown in U.S. Pat. No. 4,928,674, have employed sources of high pressure air, e.g., air tanks (50-70 psi) to rapidly inflate the vest. High pressure air was believed to be necessary to provide enough force to quickly move the necessary amount of air into the vest bladder to achieve the desired rapid vest inflation and compression of the patient's thorax. Because the vest has to be cyclically inflated and deflated approximately 50 to 60 times per minute, the vest must inflate in less than 100 to 150 milliseconds. To provide high pressure air, air tanks were pressurized to levels much higher than the desired peak vest pressure. When the vest was inflated, air rushed from the tank to the vest, and the pressure in the tank dropped as the pressure in the vest rose. Computer controllers and pressure regulators monitored the vest pressure and stopped the air flow from the tank as the vest reached the desired peak pressure. The tank was repressurized by pumps, e.g., rotary-vane pumps, that continually provided highly pressurized air to the tank. The mass of air provided by the pump was relatively small as compared to the air needed to pressurize the vest. The pumps worked continually during the vest inflation and deflation cycles, and the mass of air pumped into the tank over time was sufficient to inflate the vest during the relatively-brief inflation period of the entire cycle of the vest.
The high pressure source typically included a positive displacement pump, e.g., a piston in cylinder pump, and a high pressure metal air tank. Such sources of high pressure air are capable of pressurizing the CPR-vest to a pressure that would burst the bladder, and potentially harm the patient. Accordingly, during normal operation, a CPR vest is pressurized to a much lesser pressure than the pressure of the source of pressurized air used to inflate the vest. To inflate the vest to the same pressure as the source of pressurized air could result in too much compression being applied to the patient's thorax, trauma to the patient, and damage to the CPR vest.
The high pressure air used to inflate CPR vests required safeguards to prevent over-inflation and sophisticated controllers to control the inflation and deflation cycles. While high pressure air provides rapid inflation, it presents a danger in that the vest may be over inflated. Because the forces needed for effective CPR are only slightly below the level of forces that will harm and traumatize the chest of the patient, safeguards were included in prior vest inflation systems to ensure that the vest was sufficiently pressurized for effective CPR, and to avoid applying excessive and harmful forces to a patient's chest.
To avoid over-inflation of the vest, computer controllers and complex valve systems have been used. For example, prior vest-CPR systems have included microprocessors programmed to monitor the pressure in the vest as the vest is inflated and activate the closing of pressurization valves prior to the pressure in the valve attaining the desired pressure. The activation of the pressure valves was precisely timed in advance of the vest reaching the desired pressurization because an inherent delay in activating the valves allowed additional high pressure air to continue entering the vest and further increase vest pressure. The microprocessor for the vest-CPR system was programmed to advance the valve activation command to compensate for the valve activation delay. In addition, the peak pressure at each vest inflation cycle fluctuates from cycle to cycle because of the rapid pressure rise in the vest occurring when the pressure valve is closed and because the valve is closed based on a prediction made by a microprocessor of when the vest will be fully pressurized. Due to the uncertainties in predicting when full pressurization will occur during a rapid pressure rise and the rapid pressure rise occurring in the vest at the peak pressure, the peak pressure actually attained in the vest varies from cycle to cycle.
Prior microprocessor independent controlled safety systems monitored the pressure in the CPR vest in addition to the monitoring performed by the microprocessor controlling pressurization of the vest. For example, the safety system would close the inflation valve and vent the vest if the pressure in the vest became too great. Moreover, the high pressure source required that prior vest-CPR systems have high pressure hoses and couplings that tend to be expensive and difficult to operate. Accordingly, the high pressure air needed to rapidly inflate the vest required expensive and complex control and safety systems that increased the cost of vest-CPR systems, increased the number of components and systems that could malfunction, and increased the difficulty in operating the vest-CPR system.
The efforts to develop a commercially viable vest-CPR have encountered difficulties due to the need for a high pressure air source. While high pressure air has been considered essential to rapidly inflate a vest and to allow for sufficient capacity in the inflation system for all sizes and shapes of patients, supplying and controlling air under high pressure is complex, expensive and problematic. An electric positive displacement pump, e.g., a piston or rotary vane pump, is the most common source of high pressure air in existing vest-CPR systems. Electric air pumps are one of the more expensive components of existing vest-CPR systems, often require electrical power greater than that supplied by ordinary 120 volt AC outlets, and require maintenance. The cost of an electric air pump that supplies 18-22 scfm of 50-70 psi air may be $500-$2,000 (U.S.), which adds substantially to the cost of a vest-CPR system. Most electric air pumps this powerful require a 220 volt AC connection, which are not readily available in hospital emergency rooms or other locations where vest-CPR systems are used. Accordingly, there has been a long-felt need in CPR-vest systems to solve the problems associated with high pressure air sources.